1.1 Field of the Invention
The invention relates to a character generator for displaying alphanumerical and graphic characters on a display screen, which characters are displayed simultaneously with a video picture. Such a character generator is used, for example, in television receivers for displaying operating information (On Screen Display) and for displaying information which is supplied in an encoded form by an external source (Teletext). These character generators are also used in video cameras and camera recorders for making titles and the like, and in Compact Disc players for displaying Compact Disc-recorded data.
1.2 Description of the Prior Art
A character generator for displaying characters on a display screen generally comprises a page memory for storing the character codes of the characters to be displayed. Such a character code consists of, for example, a 7-bit number which is assigned to a character in accordance with a standardized table. The character codes are applied to a pixel generator in which each character is fixed in the form of a number of pixels, for example 6 pixels horizontally and 10 pixels vertically, and which generates a color selection bit for each pixel. The successively generated color selection bits are applied to a selector with which a foreground color signal or a background color signal is selected. In this way a color is assigned to each pixel of a character, the pixels in the foreground color determining the shape of the symbol to be displayed on the display screen. For alpha-numerical symbols such as figures, letters and punctuation marks the pixel generator comprises a memory in which a matrix of n*m, in this example 6*10, color selection bits are stored for each character. For simple graphic symbols the pixel generator further often comprises a logic network which derives the color selection bits directly from the character code.
The foreground color and the background color may be fixed colors, for example black and white. They can also be programmed for each page, each row or even each character. Particularly the way in which colors in the character generator of a teletext decoder are generated may be mentioned: in fact, some of the 7bit character codes are reserved for defining a color which remains valid until the end of the row or until a new color is defined in an identical way.
A character generator further comprises means for generating a blanking signal having a value defined for each pixel, intended for blanking the normal picture signal on the display screen in order to display the generated character signal on this screen. If the picture signal is to be blanked completely, such as, for example when displaying teletext pages, the blanking signal is generated continuously. However, a video picture and a text generated by the character generator must often be displayed simultaneously. This is the case, for example, if a TV program is subtitled by means of teletext, or if operating information such as a program or channel number generated in the receiver is displayed.
Two methods are known for simultaneously displaying a picture signal and text generated by the character generator. The first method is to blank the picture signal during a "box" comprising one or more rows or parts thereof in which the characters are displayed in a foreground color as well as a background color. In this so-called "boxed" mode the readability is ensured if the two colors are sensibly chosen, but the blanking signal blanks a relevant portion of the video picture. The second method is to blank the picture signal only during the pixels which are displayed in the foreground color in response to the corresponding color selection bit. In this so-called "mixed" mode the visibility of the alphanumerical or graphic symbols is very much dependent on color and brightness differences between the characters and the video picture in the background.